Atrial parasympathetic ganglia of rats, guinea pigs, kittens and monkeys were studied. In all species the ganglia consisted of small clusters of nerve cells, nerve processes, supporting elements and blood vessels. Each ganglion was ensheathed in a connective tissue capsule. The nerve cells were large and each contained a large nucleus with 1 or more nucleoli. The cytoplasm contained much rough endoplasmic reticulum and extensive Golgi complexes. In the rat and guinea pig, and to a lesser extent in the cat, the surfaces of the perikarya were studded with short dendritic processes. The dendrites of monkey and cat neurons were much larger than those of the other species. Short and long dendrites and neuronal perikarya were all enclosed in sheath cell processes. Preganglionic nerves, interposed between the sheath cells and the surfaces of the neurons, made synaptic contact with the perikarya and the dendritic processes. Some synapses were made by axons en passant, while others involved nerve terminals. Typical cholinergic preganglionic terminals were numerous in all species. To ensure positive identification of adrenergic nerve terminals, some animals were pretreated with 5-hydroxydopamine. In these animals preganglionic terminals containing many small dense-core vesicles were seen in synaptic contact with short dendrites of the intracardiac neurons. Other presynaptic terminals contained translucent vesicles with flattened and irregular profiles, a few large dense-core vesicles and much glycogen. These did not resemble adrenergic, cholinergic or purinergic endings. The intracardiac neurons apparently serve as a final common pathway, through which at least 3 types of input may simultaneously influence the effector tissues of the heart.
